The present invention is suited for use in particular in cellular radio systems utilizing code division multiple access. Code Division Multiple Access, CDMA is a multiple access method, which is based on the spread spectrum technique, and which has been applied recently in cellular communication systems, in addition to the prior FDMA and TDMA methods. CDMA has several advantages over the prior methods, such as spectral efficiency and the simplicity of frequency planning.
In CDMA, the narrow-band data signal of the user is multiplied to a relatively wide band by means of a spreading code having a remarkably broader band than the data signal. Bandwidths used in known test systems are e.g. 1.25 MHz, 10 MHz and 25 MHz. In connection with the multiplication, the data signal spreads onto the whole of the band used. All users transmit simultaneously by using the same frequency band. An individual spreading code is used on each connection between the base station and the mobile station, and the signals of the users may be distinguished from each other in the receivers on the basis of the spreading code of each user. A CDMA transmission in accordance with prior art is illustrated in FIG. 1, in which the horizontal axis represents time and the vertical axis pseudorandom codes. Transmissions 100-106 of different users are simultaneously transmitted on the same frequency distinguished with different codes. It is also known to provide a user with more than one code, but this is made for increasing the data transfer rate. Then a user who has been provided with two spreading codes, for example, may multiply part of his symbols to be transmitted by one code and part by another code, and thus accomplish a double transmission capacity as compared with a user transmitting with one code.
Adapted filters in the receivers are synchronized with the desired signal, which is identified on the basis of the spreading code. The data signal is returned in the receiver onto the original band by multiplying it by the same spreading code as in connection with the transmission. The signals which have been multiplied by some other spreading code neither correlate nor return to the narrow band in an ideal case. They thus appear as noise from the point of view of the desired signal. It is endeavoured to select the spreading codes of the system so that they are not mutually correlated, in other words, they are orthogonal. In practice, the spreading codes are not non-correlated, and the signals of other users complicate the detection of the desired signal by distorting the received signal. This interference caused by the users for each other is termed as multiple access interference.
The quality of the transmission depends on the number of users, in particular in the direction of transmission from the base station to the terminal equipment. The more users there are in the system, the higher is the power the base station must use for transmission. This generates interference to surrounding cells.
It is previously known to carry out interference cancellation for the received signal with the aid of which cancellation e.g. the quality of the received signal may be improved. Heretofore, plans have been made to use interference cancellation methods mainly at the base stations, since the processing required at the terminal equipment has been too complicated to implement.